Magnetic disks and disk drives are conventionally employed for storing data in magnetizable form. Preferably, one or more disks are rotated on a central axis in combination with data transducing heads positioned in close proximity to the recording surfaces of the disks and moved generally radially with respect thereto. Magnetic disks are usually housed in a magnetic disk unit in a stationary state with a magnetic head having a specific load elastically in contact with and pressed against the surface of the disk. Data are written onto and read from a rapidly rotating recording disk by means of a magnetic head transducer assembly that flies closely over the surface of the disk. Preferably, each face of each disk will have its own independent head.
In a magnetic media, digital information (expressed as combinations of “0's” and “1's”) is written on tiny magnetic bits (which themselves are made up of many even smaller grains). When a bit is written, a magnetic field produced by the disc drive's head orients the bit's magnetization in a particular direction, corresponding to either a 0 or 1. The magnetism in the head in essence “flips” the magnetization in the bit between two stable orientations.
Magnetic thin-film media, wherein a fine grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are generally classified as “longitudinal” or “perpendicular,” depending on the orientation of the magnetic domains of the grains of the magnetic material. In longitudinal media (also often referred as “conventional” media), the magnetization in the bits is flipped between lying parallel and anti-parallel to the direction in which the head is moving relative to the disc. In perpendicular media, the magnetization of the disc, instead of lying in the disc's plane as it does in longitudinal recording, stands on end perpendicular to the plane of the disc. The bits are then represented as regions of upward or downward directed magnetization (corresponding to the 1's and 0's of the digital data).
FIG. 1 shows a disk recording medium and a cross section of a disc showing the difference between longitudinal and perpendicular recording. Even though FIG. 1 shows one side of the non-magnetic disk, magnetic recording layers are sputter deposited on both sides of the non-magnetic aluminum substrate of FIG. 1. Also, even though FIG. 1 shows an aluminum substrate, other embodiments include a substrate made of glass, glass-ceramic, NiP/aluminum, metal alloys, plastic/polymer material, ceramic, glass-polymer, composite materials or other non-magnetic materials.
Efforts are continually being made to increase the areal recording density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (SMNR) of the magnetic media. To continue pushing areal densities and increase overall storage capacity, the data bits must be made smaller and put closer together. However, there are limits to how small the bits may be made. If the bit becomes too small, the magnetic energy holding the bit in place may become so small that thermal energy may cause it to demagnetize over time. This phenomenon is known as superparamagnetism. To avoid superparamagnetic effects, disc media manufacturers have been increasing the coercivity (the “field” required to write a bit) of the disc. However, the fields that can be applied are limited by the magnetic materials from which the head is made, and these limits are being approached.
Newer longitudinal recording methods could allow beyond 140 gigabits per square inch in density. A great challenge however is maintaining a strong signal for the bits recorded on the media. When the bit size is reduced, the signal is decreased, making the bits more difficult to detect, as well as more difficult to maintain stable after recording information.
One of the key challenges to extending magnetic recording technology beyond the currently achieved 35-100 Gbit/in2 areal densities is to improve the signal to noise ratio by media noise suppression. It is well known that the longitudinal scaling approach, which is to reduce the media grain surface area in proportion to the bit cell surface area, is limited by the onset of super-paramagnetic instability, which is explained above. Several options exist to avoid this problem. Traditionally, one has compensated the loss in grain volume by increasing the magnetic hardness (anisotropy); however, the write field of the recording head limits this approach. Anti-ferromagnetically coupled (AFC) dual-layer media have offered some relief by increasing the effective media thickness and consequently grain volume without compromising writability. The extendibility of this approach, however, appears to be rather limited. On the other hand, with three times smaller grain diameters, from currently about 9 nm down to about 3 nm, and correspondingly about ten times higher areal densities could be feasible if writing magnetically much harder materials, such as face-centered-tetragonal FePt alloys, can be accomplished.
Thus, one approach to enhancing the areal density in magnetic recording is to reduce the grain-count per bit. This approach could lead to higher media noise and would, among many other things, require significantly improved channel detectors. On the media side, the grain size distribution needs to be trimmed below 10% (sigma over mean), in order to reach grain-counts as low as five to ten grains per bit. Current state-of-the-art sputtered media have grain size distributions of about 25%. Thus, it remains an open challenge to bring out the required improvements in grain size and grain size distribution using physical, thin film sputtering processes. This challenge has been addressed in this invention by the use of a magnetic recording media having a structure substantially similar to that of anti-ferromagnetically coupled (AFC) media.